Kinetics of particle cracking

It is appropriate to emphasize again that mechanisms formulated on the basis of kinetic observations should, whenever possible, be supported by independent evidence, including, for example, (where appropriate) X-ray diffraction data (to recognize phases present and any topotactic relationships [1257]), reactivity studies of any possible (or postulated) intermediates, conductivity measurements (to determine the nature and mobilities of surface species and defects which may participate in reaction), influence on reaction rate of gaseous additives including products which may be adsorbed on active surfaces, microscopic examination (directions of interface advance, particle cracking, etc.), surface area determinations and any other relevant measurements. 

Any study of cumene cracking made above 400°C. with commercial cracking catalyst of conventional activity levels, with particle size of approximately 1 mm. or greater (as is the usual practice in integral reactor studies), and with pure cumene will almost certainly be diffusion limited. If this is not the case, it is by virtue of the fact that the cumene contains inhibitors which reduce the reaction rate and, of course, alter the kinetics of the reaction. This can be seen by considering the diffusion results in Table IV and the discussion of effect of inhibitors. 

In the study to characterize the dissolution kinetics of saccharin as a sweetener excipient, it was found that as dissolution continues, cracks appeared on the surface resulting in faster dissolution. Therefore Dr of Brownian and non-Brownian particles cannot be restricted to the range of 1-3. Using Dr as a parameter to distinguish dissolution mechanism is not appropriate because Dr depends on variety of parameters and not only on the type and size of the particles. 

Carbon-based catalysts and in particular their kinetics have been intensively studied [41 3], because they should reduce the disadvantages related to metal-based catalysts. Carbon materials are more available, have the potential of cost reduction, do not require periodic regeneration because it is not necessary to separate the carbon-product from the catalyst. The fluidised bed reactor technology represents the optimal choice for this kind of hydrocarbon cracking process as it can withdraw the carbon particles evermore, permitting a reliable storage of produced carbon for further use [44 46]. A novel technological solution aimed to improve activity and stability of carbon catalysts has been recently proposed [47]. The presence of small amount of O2 in an autothermal approach seems to be the best solution to minimize CO2 emissions in the overall process. 

Crack Stability. At low test speeds, stable crack growth with an extended stress-whitened plastic zone and crack blunting occur by the same mechanisms as those involved in the kinetics of the plastic zone, namely, rubber cavitation followed by shear deformation of the matrix. The ability of the matrix to shear is controlled by its relaxation behavior, which therefore determines its plasticity and the deformation imposed on rubbery particles distant from the notch. 

Upon impact, this kinetic energy can be used to shatter or deform the abrasive particle, crack or deform old paint, or chip away rust. The behavior of the abrasive, as that of the old coating, depends in part on whether it favors plastic or elastic deformation. 

In the present study, our goal was to clarify the kinetics of strength recovery by self-crack-healing as a function of Pq2- To accomplish this, we investigated the crack-healing behavior of alumina/15 vol. % SiC particle composites under several f 02 atmospheres. 

Treatment with UV-ozone involves generation of reactive atomic oxygen in a sequence of photochemical processes [45-48]. It is less effective than plasma treatment due to the lack of particles of high kinetic energy, but allows for a deeper modification of the surface without cracking or mechanical weakening of the treated PDMS. Air-filled... 

Several different test reactions have been suggested to evaluate the catalytic activity of an acid catalyst as a measure of the number and strength of the active sites. The ideal test reaction is experimentally easy, fast, reproducible, requires only a small amount of catalyst, has simple kinetics, and should show little deactivation. It should also not be diffusion limited and affected by the particle or crystal size. While no one reaction fits all these criteria perfectly, we and apparently others - find that hexane cracking comes closer to the ideal than most other reactions. 

Column II of Table IV gives, for each particle size, the rate of cracking of cumene above which diffusion phenomena will influence the observed kinetics for the silica-alumina catalyst described. Column III of Table IV gives the temperature at which these rates are observed for very pure cumene. This temperature was determined from the experi-... 

It is a principal conclusion that in large wood chip pyrolysis, experimental product distribution versus time behavior cannot be predicted with simple first order kinetics for any components. This deficiency is pronounced as particle size increases and the proposed secondary reactions (of tar) add to the primary products. It is speculative but interesting to suppose that cracking reactions occur in the char which is consistent with the greater and delayed appearance of unsaturated hydrocarbon peaks for experiments on longer pellets. 

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